It is known to use additive manufacturing technology and techniques, together with polymer powders, to manufacture high-performance products having applications in various industries (e.g., aerospace, industrial, medical, etc.).
SLS is an additive manufacturing technique that uses a laser to fuse small particles of plastic, metal (direct metal laser sintering), ceramic, or glass powders into a mass having a desired three-dimensional (3-D) shape. The laser selectively fuses the powder material by scanning cross-sectional layers generated from a 3-D digital description of the desired object onto the top layer of a bed of the powder material. After a cross-sectional layer is scanned, the powder bed is lowered by one-layer thickness in a z-axis direction, a new top layer of powder material is applied to the powder bed, and the powder bed is rescanned. This process is repeated until the object is completed. When completed, the object is formed in a “cake” of unfused powder material. The formed object is extracted from the cake. The powder material from the cake can be recovered, sieved, and used in a subsequent SLS process.
Polyaryletherketones (“PAEK”) are of interest in the SLS process because parts that have been manufactured from PAEK powder or PAEK granulates are characterized by a low flammability, a good biocompatibility, and a high resistance against hydrolysis and radiation. The thermal resistance at elevated temperatures as well as the chemical resistance distinguishes PAEK powders from ordinary plastic powders. A PAEK powder may be a powder from the group consisting of polyetheretherketone (“PEEK”), polyetherketoneketone (“PEKK”), polyetherketone (“PEK”), polyetheretherketoneketone (“PEEKK”) or polyetherketoneetherketoneketone (“PEKEKK”).
PEKK powders are of particular interest in the SLS process because objects that have been manufactured from PEKK powders via SLS have demonstrated not only the above characterizations but also superior strength relative to other PAEK materials. Furthermore, PEKK powders are unique in the SLS technique because unused PEKK powder can be recycled in subsequent SLS processes and the resultant pieces exhibit increased strength as compared to similar parts made with virgin powder.
In order to prepare the PEKK powder, raw PEKK is milled to form a PEKK powder. The grinding step can be performed using known grinding techniques, for example jet milling, by companies such as Aveka, Inc. of Woodbury, Minn., USA. Upon completion of the grinding step, the powder particles typically range in size from 8 μm to 160 μm, as determined by post-milling measurement.
A disadvantage of performing SLS on powder compositions with PEKK is that it is not possible to build objects when fine particles are included in the feedstock. Fine particles, typically described in this context, have a diameter of 30 μm or less and are sometimes referred to as fines. It is not possible to use the fines in SLS because they inhibit the application of powder in the SLS machine. For example, the fines may cause pilling, sticking, and other forms of fouling in steps of the SLS process in which smooth flowing powder are required. Therefore, it is understood that it is not possible to operate the SLS machine to build parts using milled powder that includes fine particles.
It is known to overcome the aforementioned disadvantages associated with milled SLS feedstocks by removing the fines from the feedstock prior to use in the SLS procedure. The removal of fine particles can be achieved by identifying a cut off size, for example 30 μm, and sieving particles below this identified value from the milled feedstock via an air classification or other method so as to remove the problematic particles from the lot.
US Publication No. 2015/0328665 to Hexcel Inc. for a Method for Preparing Fine Powders for Use in Selective Laser Sintering Processes acknowledges that fine powders cannot be used in the SLS process and further discloses the methods of eliminating fines from the feedstocks prior to performing SLS.
US Publication No. 2006/0134419 to Monsheimer et al. for Use of Polyarylene Ether Ketone Powder in a Three-Dimensional Powder-Based Moldless Production Process, and Moldings Produced is directed to a powder containing a porous PAEK whose BET surface area is from 1 to 60 m2/g in the SLS process. Monsheimer teaches that for better processability in a rapid prototyping or rapid manufacturing system, the fraction of particles smaller than 30 μm should eliminated from the milled particle feedstock via sifting. U.S. Pat. No. 8,795,833 to Dallner et al for Polyoxymethylene Laser Sintering Powder, Process for Its Production, and Moldings Produced from This Laser Sintering Powder teaches that particles with size smaller than 30 μm (fines) are removed from the ground product prior to SLS. US Publication No. 2017/0028632 to Cox for Powder Bed Fusing Thermoplastic Polymers explains that fines that are generated during the milling process cannot be used in SLS. US Publication No. 2018/0009982 to Steel for compounded copolyimide powders for use in SLS teaches removal of the fines fractions for improved flowability in the SLS process.
Another disadvantage resulting from the need to remove fine particles from the polymer feedstock prior to SLS is that it significantly increases the cost of performing SLS because there is no commercial demand for the fine particles separated from the feedstock during the sieving process and therefore they are considered waste. The removal of fine particles prior to SLS reduces the yield of materiel generated during the milling process, sometimes by up to 33%, and thus increases the cost of parts made via SLS.